Impact of media coating on simultaneous manganese removal and remineralization of soft water via calcite contactor

• Published in: Water research (Oxford) 2019-09, Vol.161, p.601-609
• Main Authors: Pourahmad, Hamed, Haddad, Maryam, Claveau-Mallet, Dominique, Barbeau, Benoit
• Format: Article
• Language: English
• Subjects: Calcite contactor; Groundwater supplies; Manganese removal; PHREEQC; Soft water remineralization; Sorption
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2019.06.037

The aim of this study was to investigate the negative impact of a newly-formed manganese (Mn)-layer on calcite dissolution in the long-term operation of a calcite contactor. Simultaneous removal of Mn and remineralization of soft water in an up-flow calcite contactor was conducted and led to a progressive loading of Mn into the calcite matrix. The calcite contactor demonstrated high Mn removal; however, the hardness release decreased from 32 to 20 mg CaCO3 L−1 after 600 h of operation on a high Mn concentration (5 mg L−1) feed. For an elevated Mn concentration (i.e. 5 mg Mn L−1) in the feed water, the coated layer was mainly composed of Mn which inhibits the mass transfer from the calcite core to the liquid phase. The superficial layer was identified as 5.2% Mn oxides (MnOx) by X-ray photoelectron spectroscopy (XPS). Therefore, it is postulated that Mn removal starts with an ion exchange sorption reaction between soluble Mn2+ from aqueous phase and Ca2+ from the CaCO3 matrix which is followed by a slow recrystallization of MnCO3 into MnO2. On the other hand, when the Mn content in the feed water was lower (i.e. 0.5 mg Mn L−1), a considerably lower amount of MnOx was detected on the coated media. For all the examined conditions, the formation of this coating improved Mn removal due to the autocatalytic nature of the adsorption/oxidation of dissolved manganese by MnOx. A mechanistic model based on calcite dissolution and the progressive formation of a MnO2 layer was implemented in PHREEQC software to predict the reduction in hardness release expected in long-term operation. The model was calibrated with experimental data and resulted in realistic breakthrough curves. In order to accurately predict the pH of the effluent stream, a slow-rate recrystallization of MnCO3 into MnO2 was implemented (compared to the fast precipitation of MnO2 or the absence of MnO2 formation). [Display omitted] •Calcite removed >95% of Mn2+, effluent concentration reached ≤45 μg Mn2+L−1.•At 5.0 mg Mn2+ L−1, Mn integrated into calcite and then recrystalized to MnOx.•Newly-formed MnOx layer deteriorates calcite dissolution.•At 0.5 mg Mn2+L−1, sorbed Mn incorporates into calcite matrix.